A known type of sound reinforcement system makes use of “line sources” of loudspeakers. Each loudspeaker comprises an electro-acoustic driver housed in some form of enclosure with the diaphragm of the driver radiating from a front of the enclosure directly into the environment. The individual loudspeakers are arranged as vertical one-dimensional arrays. Such arrays radiate sound throughout a very large angle in a horizontal plane. A typical line source radiates sound averagely through an angle of 90° and up to 130° in a horizontal plane.
An advantage of such an arrangement is, theoretically, even coverage within a large included angle. However, a disadvantage is that, if more than one such line source is needed in order to provide sufficient sound pressure levels at a venue, several line sources must be used but must be spaced apart by a sufficiently large distance so as to avoid undesirable interference with each other. Mutual spacings of greater than about 30 feet (about 10 metres) are required so that undesirable interference effects are reduced to an acceptably low level, for example below normal audibility to listeners. Because of the wide dispersion pattern, it is impossible to avoid destructive interference when a listener is located away from a centre vertical plane between a pair of line sources. It is therefore impossible to achieve an increase in sound pressure level simply by adding another line source adjacent an existing line source.
The vertical dispersion of a line source of this type is very narrow. To achieve sufficient vertical dispersion, the angle in a vertical plane between the sound propagation axes of adjacent pairs of loudspeakers in the array is increased in small increments of increasing amount towards the bottom of the array. This results in a graduated curve of “J” shape so that people located nearly underneath the line source also receive sound.
The increase in vertical coverage provided by such a curved array is only achieved in and adjacent a vertical plane containing the propagation axes of the loudspeakers of the array. At locations away from this plane, the effective angle between adjacent pairs of sources is reduced. This continues until, for locations facing the side of the array at 90° to the vertical plane, the vertical angle between adjacent loudspeakers becomes 0°. In other words, because of the two-dimensional nature of the curve of the array, as a listener moves off axis (i.e. away from the vertical plane containing the propagation axes) in either horizontal direction, the vertical spreading of the beam produced by the line array steadily disappears with increasing angular distance from the vertical plane. Because of the wide dispersion angle in the horizontal or generally horizontal direction, substantial sound pressures are directed in angles well away from the propagation axis of each loudspeaker. The reducing curvature with increasing off-axis angle results in beaming of sound with a higher directivity and a different perceived frequency response compared with on-axis and near-axis listening positions.
In applications where the locations of the loudspeakers of a sound reinforcement system are limited, a system based on conventional line sources may be difficult, inconvenient, or impossible to provide. For example, in applications where sound reinforcement is required throughout a relatively large area but sound reinforcement loudspeakers are restricted in location to a much smaller area, conventional line source techniques are not capable of providing an acceptable sound pressure level throughout the area. An example of such a situation is in a rectangular arena, comprising audience accommodation around the sides of a sports area, where the sound reinforcement loudspeakers may only by located at the middle of a shorter side of the rectangular arena.
U.S. Pat. No. 4,862,508 discloses an array of loudspeakers covering different frequency ranges. For example, one embodiment comprises a low frequency loudspeaker with two drivers, a mid-range loudspeaker above the low frequency loud speaker, and a tweeter above the mid-range unit. Another embodiment comprises a tweeter with mid-range loudspeakers above and below it.
U.S. Pat. No. 3,637,039 discloses a domestic loudspeaker having a two-dimensional array of drive units on a baffle.
U.S. Pat. No. 4,344,504 discloses a straight line array of horn loudspeakers opening into a waveguide which is common to all of the loudspeakers.